Gas generating device

ABSTRACT

The present invention relates to a gas generator device used in automobile safety to inflate an airbag. This device is characterized in that it comprises at least one gas source for inflating the airbag and at least one gas outlet orifice, called the first orifice, for gas to flow into the airbag, characterized in that it includes means for making the gas pass through other gas outlet orifices, called second orifices, which are distributed over the device so that it is given a neutral-thrust configuration when it is initiated, although not in service, the device also including means for closing off these second orifices when the device is in service, so that when the device is initiated, the gas passes via the first outlet orifice.

The present application is the National Stage of InternationalApplication No. PCT/FR03/00587, filed on Feb. 21, 2003, which claims thebenefit of French Application No. 02-02211, filed on Feb. 21, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to the field of automobile safety andrelates more particularly to a gas generator comprising a neutral-thrustdiffusion system.

The gas generators used in automobile safety must not become dangerousprojectiles when they are initiated, although not in service, that is tosay when they are being handled, transported or stored. It is thereforeimperative to guarantee the optimum safety level when these generatorsare outside their module. This safety level can be achieved using aneutral-thrust diffusion system. This consists, for example on gasgenerators of tubular shape, in producing balanced radial gas outletsdistributed in such a way that the forces generated upon gas liberationcancel out. The generator will thus have a neutral-thrust configurationand will present no hazard when it is initiated outside its module.However, in certain modules, in order for the airbag to inflate rapidly,it is often preferable for the gas to penetrate along a single thrustaxis. To obtain this single thrust axis directed toward the airbag, adiffuser is often added which makes it possible to channel, in a singledirection, the streams of gas escaping from the radial outlets of thegenerator. The fitting of a diffuser allows a generator with a diffusionsystem to remain in a neutral-thrust configuration. However, this typeof module is less effective than a module using an axial-outflowgenerator, that is to say one that releases the gas along a single axisin the direction of the airbag.

Patent WO 93/18942 discloses a module comprising in particular a gasgenerator fixed to a specific structure and an airbag that can beinflated by the gas delivered by this generator.

The generator described in that patent includes a pressurized-gasreservoir and a plurality of outlet orifices arranged and distributed soas to give the generator a neutral-thrust configuration when it is notfixed onto its structure. The module also includes obturators forclosing off certain outlet orifices of the generator so as to give it anaxial-thrust configuration when it is attached to its structure. If, forexample, the generator has two diametrically opposed orifices, one beingoriented so as to be able to take the gas directly into the airbag, theother orifice will be closed off so as to give the module anaxial-thrust configuration. In that document, the orifice used forinflating the airbag is also used to give the device a neutral-thrustconfiguration when it is initiated, although not in service. Said patentWO 93/18942 also proposes to fix the generator using the obturators. Inthis way, whenever the generator has to be removed from its module, itwill be necessary to remove the obturators and therefore open all theradial orifices. This generator will therefore always be in aneutral-thrust configuration when it is outside its module. The systemsemployed in the prior art use one or more specific parts so as to obtainaxial gas outflow in generators normally in a neutral-thrustconfiguration when they are outside their modules. These parts are, forexample, the diffuser or the obturators disclosed in patent WO 93/18942.

SUMMARY OF THE INVENTION

The object of the invention is to propose a gas generator with directaxial outflow toward the airbag, not requiring for this the use of aspecific structure, nor an obturator or diffuser, and neverthelesshaving a neutral-thrust diffusion system allowing it to represent nohazard when it is initiated outside its module.

This object is achieved by a gas generator device used in automobilesafety to inflate an airbag, comprising at least one gas source forinflating the airbag and at least one gas outlet orifice, called thefirst orifice, for gas to flow out into the airbag, this device beingcharacterized in that it includes means for making the gas pass throughother gas outlet orifices, called second orifices, when the device isinitiated, although not in service, these second orifices beingdistributed over the device so as to render it with a neutral-trustconfiguration when it is initiated, although not in service, the devicealso including means for closing off these second orifices when thedevice is in service, so that when the device is initiated the gaspasses via the first outlet orifice.

In one embodiment, the device comprises a first portion comprising thegas source and a second portion having the shape of a cylindrical tubethat includes a ferrule onto which the airbag can be fitted, this tubecomprising a central channel having an internal profile that defines agas outflow section, for outflow in a given flow direction between oneend, called the upstream end, which is connected to the gas source and aclosed end, called the downstream end, which constitutes a solid endsection of the tube, the first orifice being formed through this solidsection and the tube also having a side wall on which the secondorifices are formed radially.

According to a first embodiment, the means for making the gas passthrough the second orifices when the device is initiated, although notin service, consist of a particular internal profile for flow in thetube, which comprises, upstream of the second orifices, along the flowdirection, a flow section that decreases down to a minimum flow sectionof nonzero given diameter forming a restriction, extended approximatelyin the same radial plane by a flow section of diameter greater than thediameter of the minimum section, which is itself extended by anincreasing flow section so as to form, with this particular internalprofile, a Venturi-effect system, the difference between the twodiameters being defined so as to leave an opening near an annularpassage connected to the first orifice, this first orifice being formedby what is called an axial nozzle oriented in the direction of theairbag, said nozzle being formed in the side wall of the tube and, atone of its ends, passing through the end section of the tube.

According to one feature of the first embodiment, the decreasing sectionlocated upstream of the restriction is formed by a central channel of apart, this part having the shape of a truncated cone open at both itsends and having a variable defined thickness, which decreases from itsupstream end, located on the same side as the upstream end of the tube,toward its downstream end, located on the same side as the downstreamend of the tube, this part having a side wall, on a portion of which isformed, near its upstream end, a projecting circular flat, the surfaceof which is parallel to the internal wall of the tube and is fastened tosaid internal wall.

According to a second embodiment, the means for bringing the gas towardthe second orifices comprise a deflector placed so as to face the inletof a duct, this inlet constituting one end of the duct located insidethe tube, the duct, at its other end, called the outlet end, passingthrough the end section of the tube in order to emerge outside the tube,the deflector having at least one solid portion placed facing the inletof the duct, this solid portion being formed or located at a defineddistance from the inlet of the duct so as to leave a space between thesolid portion and the inlet of the duct, at least one passage beingformed around the solid portion of the deflector, this passageconstituting a reduction in the flow section in the tube between theupstream end and the downstream end.

According to a first variant of this second embodiment, the deflectorincludes, on either side of its solid portion, at least two orificesallowing the gas to pass between the upstream end and the downstream endof the deflector.

According to one feature of this first variant, the solid portion of thedeflector is domed toward the upstream end, making it possible to formthe space between the solid portion and the inlet of the duct.

According to a second variant of this second embodiment, the deflectorhas a frustoconical shape, the axis of symmetry of which coincides withthe axis of the tube and with the axis of the duct, the larger sectionof this frustoconical shape facing the inlet of the duct and having adiameter greater than or equal to the diameter of the duct at its inlet,and less than the internal diameter of the tube at this point, so as toform the passage between the upstream end and the downstream end.

According to one feature of this second variant, the inlet of the ductis flared and the deflector has means for catching onto the edge of theflared inlet of the duct.

According to one feature, the inflatable airbag has an open end with itsperimeter fitted to the ferrule, applied to the second orifices and heldagainst them so as to form the means for closing off the second orificeswhen the device is in service.

According to another feature, the link, for joining the airbag onto theferrule, formed by means of a clamping clip, can melt, which makes itpossible in the event of firing the device, to release the secondorifices and thus give the device a neutral-trust configuration.

According to another feature, the gas source comprises a reservoirconnected to or integrated with the tube and containing a pressurizedgas.

According to another feature, the gas source comprises a combustionchamber for burning at least one pyrotechnic charge.

The invention, with its features and advantages, will become moreclearly apparent on reading the description, which is given withreference to the appended drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows, in longitudinal sectional view, a first embodiment of adownstream portion of a tubular generator not in service;

FIG. 1B shows, in longitudinal sectional view, the downstream portion ofthe gas generator shown in FIG. 1A, onto which the inflatable airbag hasbeen fitted;

FIG. 2A shows, in longitudinal sectional view, a second embodiment ofthe downstream portion of the generator not in service;

FIG. 2B shows, in longitudinal sectional view, the downstream portion ofthe generator shown in FIG. 2A, onto which the inflatable airbag hasbeen fitted;

FIG. 3A shows, in longitudinal sectional view, the downstream portion ofthe generator not in service in a variant of the second embodiment;

FIG. 3B shows, in longitudinal sectional view, the downstream portion ofthe generator shown in FIG. 3A, onto which the airbag has been fitted;and

FIG. 4 shows, in longitudinal sectional view, an embodiment in which theferrule onto which the airbag is fitted is oriented at 90° to the axisof the tube.

The invention will now be described in conjunction with FIGS. 1A to 4.

To inflate an airbag for protecting the occupants of a motor vehicle, agas generator is used. Certain widely used generators are called axialoutflow generators, which means that the gas is sent into the airbagalong a single axis. The axis as defined here does not thereforecorrespond to the axis of the generator when the latter is of tubularshape, but corresponds to the axis of entry of the gas into the airbag.Other generators are called neutral-thrust generators. When theseneutral-thrust generators are of tubular shape, they have a plurality ofoutlet orifices distributed over a portion of the side wall of the tubeso that the forces generated by the gas outflow are cancelled out.

Generators with axial outflow may become dangerous projectiles if theyare initiated outside their module. However, they are very effective forinflating certain airbags.

The invention therefore consists in using an axial-outflow generatorwhen it is in its module, that is to say when it is fitted onto theairbag, which has a neutral-thrust diffusion system allowing it to be ofno hazard when it is being handled, stored or transported.

The invention may be applied in particular to tubular generators, havingany gas source (not shown) whether or not integrated into the generator.This generator may therefore have a pressurized gas reservoir or may usethe combustion of a pyrotechnic charge, or it may be of hybrid type,that is to say having a pressurized gas reservoir and a combustionchamber for burning a pyrotechnic charge. Let us take, for example, ahybrid generator comprising several gas sources. Since the inventionrelates to the discharging of the gas out of the generator, the appendedFigures show only that portion of a generator close to the end via whichthe gas is discharged. This end is called the downstream end.

The generator has a tubular shape, closed at its downstream end by whatis called end section (8, 108, 208) of the tube (1, 101, 201), the tube(1, 101, 201) having a central axis (A) and a side wall (2, 102, 202).The tube (1, 101, 201) has a central channel (10, 110, 210) with aninternal profile defining a flow section perpendicular to the axis (A)of the tube (1, 101, 201).

The gas flows through the tube in a flow direction defined by the arrowf1 in the appended Figures, between what is called the upstream end,connected to the gas source, and the downstream end of the tube (1, 101,201).

That portion of the tube (1, 101, 201) shown in FIGS. 1A to 3B has, in adefined radial plane, over a defined length L as far as its end section(8, 108, 208), a different, reduced section, thus reducing the flowsection of the tube (1, 101, 201) and constituting a ferrule for fittingthe airbag. When the generator is in service, as shown in FIGS. 1B, 2Band 3B, an airbag (3) for protecting the occupants of a vehicle isfitted to the generator. This airbag (3) is in the form of a bag andtherefore has an open end and a closed end. The open end of the airbag(3) has a perimeter designed to surround the tube (1, 101, 201) in agastight manner on its reduced section.

In a first embodiment shown in FIGS. 1A and 1B, an internal nozzle (4),inside the tube (1), is fixed to its internal wall (20), starting from atransition (12) in the section of the tube (1), over a portion of thelength L of the reduced section of the tube.

This internal nozzle (4) is, for example, fixed by crimping it onto theinternal wall (20) of the tube (1). It consists of a part having acentral channel (40), the axis of which roughly coincides with the axis(A) of the tube (1). This central channel (40) has a specified shape sothat it defines, in the tube (1), along the flow direction given by thearrow f1, a flow section that decreases up to the end of the part wherethe flow cross section is a minimum with a nonzero diameter D1. The partforming this nozzle (4) has the shape of a truncated cone having thecentral channel (40), the axis of which coincides with the axis ofsymmetry of the truncated cone. This truncated cone is open at both itsends and its thickness e varies, decreasing from the upstream end towardthe downstream end. This truncated cone has, over a portion of its sidewall (41) located near its most upstream end, over its entire periphery,a projecting flat (42), the surface of which is parallel to the internalwall (20) of the tube (1) and is fixed to said internal wall.

Lying approximately in the radial plane including the most downstreamend of the nozzle (4), this end having the minimum section of diameterD1 of the central channel (40) of the nozzle (4), the internal wall (20)of the tube (1) has a particular profile consisting of a large increase(11) in the thickness of the side wall (2) in the tube (1), andtherefore consisting of the reduction in the flow section in the tube(1) in this radial plane. The latter flow section has a diameter D2greater than the sum of the diameter D1 of the minimum section and oftwice the thickness e of the nozzle (4) in this radial plane.

Continuing along the flow direction, the tube (1) then has a flowsection (13) that increases over a certain length and then a flowsection (14) that remains constant up to the closed end section (8) ofthe tube (1). This constant flow section has a smaller diameter D3 thanthe diameter D4 of the flow section in the tube taken in the reducedsection of the tube (1).

The difference between the diameter D2 of the flow section taken at thethickness increase (11) of the side wall (2) of the tube (1) and the sumof the diameter D1 of the minimum section and of twice the thickness eof the part (4) in the radial plane defined above forms an openingtoward an annular passage (5). This annular passage (5) is formedbetween the surface of the side wall (41) of the frustoconical part (4)and that section of the tube (1) which is formed, in this radial plane,by the increase (11) in the thickness of the side wall (2) of the tube(1).

At least one nozzle (6) is formed in the side wall (2) of the tube (1)longitudinally with respect to the axis (A) of the tube (1), said atleast one nozzle (6), at one of its ends, passing through the downstreamend section (8) of the tube (1) in order to emerge outside the tube (1),in the airbag when the latter is fitted, the other end of this nozzle(6) emerging in the annular passage (5). This nozzle (6) is called anaxial nozzle as it constitutes a means for axial gas outflow in thedirection of the airbag (3) when the latter is fitted. The axis asdefined here therefore does not necessarily correspond to the axis (A)of the tube, but corresponds to the axis for gas entry into the airbag.This principle is illustrated in FIG. 4, in which the ferrule (308) usedfor fitting the airbag is, for example, oriented at 90° to the axis (A)of the tube (301) so that the axial gas outflow (306) into the airbag isalso oriented at 90° to the axis (A) of the tube (301). In the sectionalview shown in FIGS. 1A and 1B, only a single axial nozzle is visible.However, there are other axial nozzles (6) emerging in the annularpassage (5).

Radial nozzles (7) are formed through the side wall (2) of the tube (1)in the constant flow section (14) of diameter D3. These nozzles (7) aredistributed over the side wall (2) of the tube (1) so that the forcesresulting from the gas outflow through these nozzles are cancelled out.

The decreasing flow section formed by the nozzle (4) extended by theincreasing flow section (13) of the tube (1) thus forms a Venturi-effectsystem. The Venturi-effect causes the gas to accelerate in therestriction formed by the minimum flow section of diameter D1.

Referring to FIG. 1A, when the generator is initiated but is not inservice, the gas accelerates at this restriction. This accelerationtakes the gas directly into the radial tubes (7) as showndiagrammatically by the arrows f2 in FIG. 1A. In this configuration,since the opening toward the annular passage (5) leading to the axialnozzles (6) is not along the path of the gas, said gas is not dischargedvia the axial nozzles (6). Thus, no axial thrust is created when thegenerator is initiated, although not in service. The use of theVenturi-effect system therefore limits the hazards associated with theinitiation of the generator while it is outside its module.

DETAILED DESCRIPTION

Referring to FIG. 1B, when the generator is in service, the perimeter ofthe airbag (3) closes off the radial nozzles (7), thereby preventing gasoutflow via the radial nozzles (7). In this configuration, the gas,being unable to escape via the closed-off radial nozzles (7), flows viathe annular passage (5) before joining the axial nozzles (6) formed inthe side wall (2) of the tube (1) and thus reaching the inflatableairbag (3), as shown by the arrows f3 in FIG. 1B. In order for theradial nozzles (7) to be properly closed off, the perimeter of the openend of the airbag (3) is held in place by means of a clamping clip (9)that surrounds the tube (1).

In a second embodiment shown in FIGS. 2A to 3B, a duct (106, 206) passesthrough the closed end section (108, 208) of the tube (101, 201),approximately along its center, the axis of said duct being roughlycoincident with the axis (A) of the tube (101, 201) and thusconstituting axial gas outflow toward the airbag (103, 203) when thelatter is fitted. This duct (106, 206) has a first end, called theoutlet end, which emerges outside the tube (101, 201) into the airbag,when it is fitted, and a second end (105, 205), called the inlet end,which emerges inside the tube (101, 201). The inlet (105, 205) of theduct (106, 206) lies roughly level with the abovementioned transition(112, 212) in the section of the tube (1).

In a first variant of this second embodiment, shown in FIGS. 2A and 2B,a deflector (104) is placed in front of the inlet (105) of the duct(106), said deflector having a solid circular domed central portion(115), the convexity of which is oriented toward the upstream end of thetube (101). Formed in the deflector (104) on either side of this centralportion (115) are two orifices (140), the axes of which are roughlyparallel to the axis (A) of the tube (101). This deflector (104) isfixed via its perimeter (142) to the internal wall (120) of the tube (1)just upstream of the section transition (112) of the tube (101). Thesolid central portion (115) of the deflector (104) is placed in front ofthe inlet (105) of the duct (106). The diameter D6 of this solid centralportion, bounded by the orifices (140), is greater than the diameter ofthe duct (106) at its inlet (105), defined by D5 in FIGS. 2A and 2B. Theradius of curvature of the doming of the central portion (115) of thedeflector (104) is sufficient to leave a space between the inlet (105)of the duct (106) and the central portion (115) of the deflector (104).

A cylindrical part (117) is placed just upstream of the deflector (104).This part (117) is fixed via its periphery to the internal wall (120) ofthe tube (101). This part (117) has a central channel (116) offrustoconical shape, the axis of symmetry of which is roughly coincidentwith the axis (A) of the tube (101), this frustoconical shape thuscreating, in the tube (101), a change in the flow section. Thisfrustoconical channel (116) is oriented in such a way that the smallestflow section is the most upstream. The largest flow section of thiscentral channel (116) has a diameter D7 defined in such a way as to beequal to or greater than the sum of the diameter D6 of the centralportion (115) of the deflector (104) and of the diameter of each orifice(140) of the deflector (104). In this variant, orifices or nozzles (107)are formed radially through the side wall (102) of the tube (101),downstream of the section transition (112) of the tube (101). Theseorifices or nozzles (107) are distributed so as to give the generator aneutral-thrust configuration when it is initiated outside its module.These orifices or nozzles (107) may be closed off as in the firstvariant described with reference to FIGS. 1A and 1B by the perimeter ofthe airbag (3).

The central portion (115) of the deflector (104) and the frustoconicalshape of the central channel (116) of the part (117) located upstream,deflect and guide the gas toward the orifices (140) of the deflector(104).

At the orifices (140), the gas accelerates owing to the Venturi effectcreated by the restriction formed by these orifices. Upon accelerating,the gas expands around the duct (106), to be directed toward the radialnozzles (107), as indicated by the arrows f102 in FIG. 2A. If thegenerator is not in service, i.e. it is outside its module, the gasescapes via these radial nozzles (107) thus creating neutral thrustowing to the balanced distribution of the radial nozzles (107). If theseradial nozzles (107) are closed off, for example by the perimeter of theopen end of the airbag (3) as described above, the gas can then flowonly via the space existing between the central portion of the deflector(104) and the inlet (105) of the duct (106), in order thus to reach theduct (106) and be discharged into the airbag (3) as indicated by thearrow f103 in FIG. 2B. In order for the radial nozzles (107) to beproperly closed off, the perimeter of the airbag (3) is held in place bymeans of a clamping clip (109) that surrounds the tube (101).

In a second variant of this second embodiment, shown in FIGS. 3A and 3B,the inlet (205) of the duct (206) is flared. A deflector (204) is fixedto the flared edge of the inlet (205) of the duct (206), over a portionof its perimeter, for example less than one half. In this variant, saiddeflector (204) is a frustoconical part placed in such a way that itsaxis of symmetry is roughly coincident with the axis (A) of the tube(201) and the axis of the duct (206). The end (220) of smallest sectionof this part is located furthermost upstream and the end (221) oflargest section is located roughly level with the section transition(212) of the tube (201). The end (221) of largest section of this parthas a diameter D8 equal to or greater than the diameter D9 of the duct(206) taken at its inlet at the end of the flared edge. The end (221) oflargest section has a smaller diameter D8 than the inside diameter D4 ofthe tube (201) taken at the reduced section of the tube (201) so that apassage (240) is formed, around the deflector (204), between theupstream end and the downstream end of the deflector (204). Along aportion of the perimeter of its largest section (221), the deflector(204) has a projecting portion (218), allowing it to catch onto acorresponding portion of the flared edge of the inlet (205) of the duct(206). When the deflector (204) is secured to the inlet of the duct(206), a space (219) is left between the plane formed by the inlet (205)of the duct (206) and the plane containing the end (221) of largestsection of the deflector (204). The duct (206) also has, near itsoutlet, a projecting ring (222), one of the lateral surfaces of whichbears inside the tube (201) against the end section (208) of the tube(201). This ring (222) has a diameter D10 smaller than the insidediameter D4 of the tube (201) taken at its reduced section, so as not toclose off the radial nozzles. A fillet (223) is formed all around theduct (206) between the lateral surface opposite the lateral surface forbearing against the end section (208) of the tube (201). The fillet isable, for example, to guide the gas toward the radial nozzles (207).Placed upstream of the deflector (204) is a part (217) with a centralchannel identical to that described in the first embodiment withreference to FIGS. 2A and 2B.

In this second variant, the frustoconical shape of the deflector (204)makes it possible to deflect and guide the gas toward the passage (240)existing between the perimeter (240) of the deflector (204) and theinternal wall (220) of the tube (201). The passage (240) constitutes arestriction in the flow section in the tube (201). As the gas flows intothis restriction, a Venturi effect is created, that is to say the gas isaccelerated. Owing to this acceleration, the gas is directed directlyinto the radial nozzles (207) of the tube (201) that are formed in thefirst variant of this second embodiment. If these radial nozzles (207)are not closed off, the gas escapes via the radial nozzles (207) asindicated by the arrows f202 in FIG. 3A. If these radial nozzles (207)are closed off, for example by the perimeter of the airbag (3) held inplace by a clamping clip (209), the gas has no other option but to passinto the space (219) existing between the deflector (204) and the inlet(205) of the duct (206). This gas then escapes directly via the duct(206), constituting an axial gas outflow into the airbag, as indicatedby the arrow f203 in FIG. 3B. Since the projecting portion (218) of thedeflector (204), serving to fasten the deflector (204) to a portion ofthe perimeter of the flared edge of the duct (206), is solid, it alsoallows the gas to be guided into the duct (206).

In both embodiments described above, the Venturi-effect makes itpossible to direct and lead the gas into the radial nozzles (7, 107,207) and thus to give the generator a neutral-thrust configuration whenit is not in service. When these radial nozzles (7, 107, 207) are closedoff, the gas has no other option but to escape axially in the directionof the airbag (3), that is to say via the axial nozzles (6) in the firstembodiment or via the duct (106, 206) in the second embodiment.

In a variant, the clamping clip (9, 109, 209) may for example bemeltable, so as to release the radial outlets (7, 107, 707) of thegenerator when the module is fired, and thus gives the generator aneutral-thrust configuration.

It should be obvious to those skilled in the art that the presentinvention allows embodiments in many other specific forms withoutdeparting from the field of application of the invention as claimed.Consequently, the embodiments presented must be regarded by way ofillustration; however, they may be modified within the field defined bythe scope of the appended claims and the invention must not be limitedto the details given above.

1. A gas generator device used in automobile safety to inflate an airbag(3), comprising at least one gas source for inflating the airbag (3) andat least one gas outlet orifice (6, 106, 206), called the first orifice,for gas to flow out into the airbag (3), comprising a first portioncomprising the gas source, a second portion comprising a side wall (2,102, 202) through which other gas outlet orifices (7, 107, 207), calledsecond orifices are formed and a flow guide for making the gas passexclusively through the second orifices (7, 107, 207) when the device isinitiated, although not in service, the second orifices (7, 107, 207)being separate from the first orifice (6, 106, 206) and distributed overthe side wall (2, 102, 202) of the device so as to render it with aneutral thrust configuration in which forces resulting from the gasoutflow through these second orifices are canceled out, when it isinitiated, although not in service, the device further comprising aclosing mechanism that closes off the second orifices when the device isin service, so that when the device is initiated the gas passes via thefirst outlet orifice (6, 106, 206).
 2. The device as claimed in claim 1,wherein the second portion has the shape of a cylindrical tube (1, 101,201) that comprises a ferrule onto which the airbag (3) can be fitted,the tube (1, 101, 201) further comprising a central channel (10, 110,210) having an internal profile that defines a gas outflow section, foroutflow in a given flow direction (f1) between one end, called theupstream end, which is connected to the gas source and a closed end,called the downstream end, which constitutes a solid end section (8,108, 208) of the tube (1, 101, 201), the first orifice being formedthrough this solid end section and the second orifices being formedradially through the side wall (2, 102, 202) of the tube (1, 101, 201).3. The device as claimed in claim 2, wherein the flow guide for makingthe gas pass through the second orifices when the device is initiated,although not in service, comprises a particular internal profile forflow in the tube (1), which comprises, upstream of the second orifices,along the flow direction (f1), a flow section that decreases down to aminimum flow section of nonzero given diameter (D1) forming arestriction, extended approximately in the same radial plane by a flowsection of diameter (D2) greater than the diameter (D1) of the minimumsection, which is itself extended by an increasing flow section (13) soas to form, with this particular internal profile, a Venturi-effectsystem, the difference between the two diameters (D1, D2) being definedso as to leave an opening near an annular passage (5) connected to thefirst orifice formed by an axial nozzle (6) oriented in the direction ofthe airbag (3), the nozzle being formed longitudinally in the side wall(2) of the tube (1) and, at one of its ends, passing through the endsection (8) of the tube (1).
 4. The device as claimed in claim 3,wherein the decreasing section located upstream of the restriction isformed by a central channel (40) of a part (4), the part (4) having theshape of a truncated cone open at both its ends and having a variabledefined thickness (e), which decreases from an upstream end, located onthe same side as the upstream end of the tube (1), toward a downstreamend, located on the same side as the downstream end of the tube (1), thepart (4) having a side wall (41), on a portion of which is formed, nearthe upstream end, a projecting circular flat (42), the surface of whichis parallel to the internal wall (20) of the tube (1) and is fastened tothe internal wall.
 5. The device as claimed in claim 2, wherein the flowguide for bringing the gas toward the second orifices further comprisesa deflector (104, 204) placed so as to face the inlet (105, 205) of aduct (106, 206), the inlet (105, 205) constituting one end of the duct(106, 206) located inside the tube (101, 201), the duct (106, 206), atanother end, called the outlet end, passing through the end section(108, 208) of the tube in order to emerge outside the tube (101, 201),the deflector (104, 204) having at least one solid portion placed facingthe inlet of the duct (106, 206), the solid portion located at a defineddistance from the inlet of the duct so as to leave a space between thesolid portion and the inlet (105, 205) of the duct (106, 206), at leastone passage being formed around the solid portion of the deflector (104,204), the passage (140, 240) constituting a reduction in the flowsection in the tube between the upstream end and the downstream end. 6.The device as claimed in claim 5, wherein the deflector (104) includes,on either side of its solid portion (115), at least two orifices (140)allowing the gas to pass between the upstream end and the downstream endof the deflector (104).
 7. The device as claimed in claim 6, wherein thesolid part (115) of the deflector is domed toward the upstream end,providing space between the solid portion (115) and the inlet of theduct (106).
 8. The device as claimed in claim 5, wherein the deflector(204) has a frustoconical shape, the axis of symmetry of which coincideswith the axis (A) of the tube (201) and with the axis of the duct (206),the larger section (221) of the frustoconical shape facing the inlet(205) of the duct (206) and having a diameter (D8) greater than or equalto the diameter (D9) of the duct (206) at its inlet, and less than theinternal diameter (D4) of the tube (201) at this point, so as to formthe passage (240) between the upstream end and the downstream end. 9.The device as claimed in claim 8, wherein the inlet (205) of the duct(206) is flared and in that the deflector (204) comprises a connecterthat fixes the deflector onto the edge of the flared inlet (205) of theduct (206).
 10. The device as claimed in claim 2, wherein the inflatableairbag (3) has an open end with a perimeter fitted to the ferrule,applied to the second orifices and held against the orifices so as toform the flow guide for closing off the second orifices when the deviceis in service.
 11. The device as claimed in claim 10, wherein the linkfor joining the airbag (3) onto the ferrule is formed by a clamping clip(9, 109, 209).
 12. The device as claimed in claim 11, wherein the linkfor joining the airbag (3) onto the ferrule formed by a clamping clip(9, 109, 209) is meltable thereby permitting the release of the secondorifices and providing the device a neutral thrust configuration whenthe device is fired.
 13. The device as claimed in claim 1, wherein thegas source comprises a reservoir connected to or integrated with thetube (1, 101) and containing a pressurized gas.
 14. The device asclaimed in claim 1, wherein the gas source comprises a combustionchamber for burning at least one pyrotechnic charge.